Hi Slovenia,
I would seriously suggest that you check the ferry availabilty at the isle of man Steam Packet company website before getting toooo excited. TT and Manx Grand Prix are usually booked solid both for arrivals and departures.
This years TT is on the first two weeks of June and NOT last week of May/ first week of June.
Good Campsites at Glenlough farm just outside Douglas and at Peel - Again could be booked out.
Dennis